Since the 1930s, the idea that creatures might advertise their " fitness " as mates through symmetry has been around. It rests on the notion that factors ranging from bad genes to coming off worse in too many fights will lead to asymmetries others can see. And over the past few years, experiments with animals and humans seemed to support the idea that symmetry in features such as plumage and facial characteristics influences mate selection.
But no one had actually checked that the animals in these experiments can actually see the subtle differences in symmetry, typically of between 1 and 2 percent. Now the first such experiment has been carried out. The results are worrying for advocates of symmetry detection as a powerful factor in mate selection.
John Swaddle, an ethnologist at the University of Bristol, performed the experiment using wild starlings, which have excellent eyesight. He trained them to hit keys marked with bar patterns with varying levels of asymmetry in order to receive food rewards. This showed the starlings could easily detect asymmetries of between 5 and 10 percent. But at between 1 and 2 percent, their performance plummeted to no better than random guessing.
" This suggests that the levels of asymmetry that birds encounter in nature will often be just too small to be detected, " says Swaddle. He says experiments that linked small asymmetries to fitness may have produced misleading correlations, and says the only direct evidence that birds use asymmetry as a cue to mate fitness involves levels of asymmetry of at least 10 percent. " I think signaling by asymmetry will probably only occur when species show such very large asymmetries—and this doesn’t occur that often. "
This raises questions about research suggesting that humans are influenced by visual asymmetry in their choice of partner. Michael Butt of the Perception Laboratory at the University of St. Andrews, who has carried out such experiments, concedes that no one has ever checked if humans have a threshold to asymmetry detection. He thinks a conclusive test of this would probably involve manipulating complex three-dimensional images: " It would be a very difficult experiment to do "
Burt adds that the abilities of animals to detect asymmetry in bar patterns may not reflect their talent for spotting asymmetries in body shape. " The visibility of an asymmetry may well depend on its type, " he says. Swaddle agrees, and is planning further experiments. " But I suspect that asymmetry is used as a visual cue less often than most people appear to presume, " he warns.
John Swaddle’s experiment suggests that（）
A. asymmetry is seldom big enough
B. fitness may cause incorrect relations
C. birds occasionally rely on symmetry in mate selection
D. random guessing is sometimes as good as asymmetry detection
参考答案：A
解析：
推理题。第四段最后一句提到：“I think signaling…and this doesn’t occur that often.”(我认为只有当物种显示出非常大的非对称性时，非对称性才会成为配偶选择时的信号，但是这种情况很少发生)所以A项正确。第四段第二句提到“experiments that linked small asymmetries to fitness may have produced misleading correlations”(那些将细微的非对称性与生物择偶时考虑的适合性联系起来的实验让人们误以为两者间具有相关性)，B项是对此的曲解；John Swaddle的实验只是考察了鸟类对非对称性有多大的感知能力，没有讨论鸟类在择偶时是否依赖对称性，故排除C项；文章提到“对于只有1%到2%之间的非对称性，欧掠鸟的判断力则急剧下降到近乎随意乱猜”，这只是对感知很小的非对称性而言的，不能笼统地说随意乱猜的效果与感知非对称性的效果一样，故排除D项。